1. Field of the Invention
The present invention relates to a fuel cell system and a method of controlling the same capable of preventing anode flooding due to a temperature difference between a stack and reformate upon starting the fuel cell system.
2. Description of the Related Art
In general, a fuel cell is a system that directly converts chemical energy into electric energy by the electro-chemical reaction of fuel and an oxidant. The fuel cell has been spotlighted as next-generation power generation technology, since it does not need a combustion process or a driving apparatus like an existing turbine generator and has high power generation efficiency as well as does not generate environmental problems such as air pollution, vibration, noise, etc. The fuel cell may be categorized into a phosphoric acid fuel cell, an alkaline fuel cell, a polymer electrolyte membrane fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, etc. according to the kind of electrolyte. The respective fuel cells are basically operated in the same principle, but have different types of fuels, operation temperatures, catalysts, electrolytes, etc. These fuel cells have been researched and developed for various uses, such as an industrial use, a household use, a leisure use, etc. In particular, some fuel cells have been researched and developed as a power supply of a transportation means, such as vehicles, ships, etc.
Among others, the polymer electrolyte membrane fuel cell (PEMFC), which uses a solid polymer membrane, not a liquid electrolyte, as an electrolyte, has advantages of high output characteristics, low operating temperature, and a rapid starting and response characteristics, as compared to the phosphoric acid fuel cell, and is widely applicable to a portable power, portable electronic devices, transportation, such as a car, a yacht, as well as a distributed power, such as a stationary power generating stations used in a house and a public building, etc.
The polymer electrolyte membrane fuel cell system can be largely represented by two component groups, that is, a stack and a system and an operation part. The stack directly generates electricity by the electro-chemical reaction of fuel and an oxidant, and includes an anode electrode catalyst, a cathode electrode catalyst, and a membrane-electrode assembly of an electrolyte inserted between these electrode catalysts. Also, the stack may be manufactured by a stack of a plurality of membrane-electrode assemblies. In the case of the stack-type stack, separators are disposed between the membrane-electrode assemblies. The system and operation part includes a fuel supplier, an oxidant supplier, a heat exchanger, a power converter, a controller, etc. to control the operation of the stack.
Temperature change in the aforementioned polymer electrolyte membrane fuel cell is initiated during an initial starting of the system. Since the stack does not start the electro-chemical reaction of fuel and oxidant just after the system is started, the temperature of the stack is lower than the temperature at normal operation state. Electric energy and heat are generated with time from the stack through the electro-chemical reaction of fuel and oxidant after the start of the operation. The temperature of the stack gradually rises due to the generated heat during the operation.
Meanwhile, if a reformate, which is heat-exchanged at the normal operation temperature, is supplied to the anode of the stack upon starting the system, because the stack usually has lower operation temperature than the normal operation temperature, a considerable amount of steam included in the reformate is condensed in the inside of the stack. The condensation of steam in the inside of the stack leads to a problem of anode flooding, thereby hindering the normal starting and operation of the stack.
In particular, water condensed in the inside of the stack is likely to be collected in the lower of the stack. In this case, some cells in the stack may sink under water. These cells generate reverse voltage to the stack to hinder the starting of the stack. Furthermore, if the system with the aforementioned problems is repeated, the stack performance can be suddenly degraded.